Biaxially oriented polypropylene films are typically used for packaging, decorative, and label applications and often perform multiple functions. In a lamination, they provide printability, transparent or matte appearance, or slip properties. The films sometimes provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. The films sometimes provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or by laminating.
However, in recent years, interest in “greener” packaging has been strongly developing. Interest in packaging materials based on biologically derived polymers is increasing due to concerns with non-renewable resources, waste production, raw materials, and the production of greenhouse gases. Biodegradable polymers help alleviate the growing environmental problem of the production of an excessive amount of plastic waste. Non-biodegradable plastic waste requires years to decompose and includes an ever increasing volume fraction of the waste present in landfills. Also, it is believed that bio-based polymers, once fully scaled up, will help reduce reliance on petroleum and thereby reduce the production of greenhouse gases due in part to their sustainably-sourced feedstocks (i.e. plant-sourced).
Bio-based polymers such as polylactic acid, which is derived from corn starch and thus can be considered to be derived from a renewable resource, is one of the more popular and commercially available materials available for packaging film applications. However, due to the commercial expenses compared to traditional polymers and the difficulties that can arise in the processing of these bio-polymers to form a product comparable to or matching that of existing products, there has been little commercial success. Many compositions involving these polymers exhibit limited quality, processability, degradability, or some combination thereof.
For such a bio-based polymer to be fit-for-use for many snack food packaging applications, it is desirable that the bio-based polymer film match as many of the attributes as possible, and therefore exhibit the level of quality and performance, that BOPP is well-known for such as heat sealability, printability, controlled COF, metallizability, barrier, etc. In particular, for high barrier packaging, metallized oriented PLA films should demonstrate good oxygen and moisture barrier properties. For metallized oriented PLA in particular, high oxygen barrier property is generally easily achieved due to the polar nature of PLA, which provides good hydrogen-bonding of the polymer molecules. However, this polar nature tends to be detrimental for achieving high moisture barrier. Without being bound by any theory, the thought is that water molecules—being polar—may more easily migrate through a polar polymer film than a non-polar polymer film. In order to achieve a useful protection of snack food products from staleness and/or rancidity, and to ensure a reasonably adequate shelf-life, it is preferable to have a moisture barrier property of at least about 1.0 g/m2/day or better, and more preferably, to have a moisture barrier property of about 0.50 g/m2/day or better, at 38° C. and 90% RH. It is preferable to have an oxygen barrier of at least about 46.5 cc/m2/day, and more preferably 31 cc/m2/day or better, at 23° C. and 0% RH.
Many products currently on the market at the time of this writing do not provide satisfactory moisture barrier properties. For example, Celplast Metallized Products, Ltd.'s Enviromet™ high barrier metallized PLA film data sheet describes a product that exhibits an excellent oxygen barrier of 6.2 cc/m2/day (at 23° C., 50% relative humidity or RH) but a relatively poor moisture barrier of 3.1 g/m2/day (at 38° C., 90% RH) as compared to typical metallized biaxially oriented polypropylene films. (High barrier metallized BOPP such as Toray Plastics (America), Inc.'s PWX3 product typically demonstrates oxygen barrier of 15.5 cc/m2/day (23° C., 0% RH) and moisture barrier of 0.155 g/m2/day (38° C., 90% RH)). Another manufacturer of barrier PLA film, Alcan Packaging Inc., produces a silicon oxide coated PLA film under the tradename Ceramis® whose data sheet shows an oxygen barrier of 7.75 cc/m2/day (23° C., 50% RH) and moisture barrier of 7.75 g/m2/day (38° C., 90% RH). Biofilm S.A. promotional literature (such as presented at the “Innovation Takes Root” conference hosted by NatureWorks LLC at Las Vegas, Nev. Sep. 16-18, 2008) discusses transparent barrier PLA films demonstrating moisture barrier of 3-10 g/m2/day (38° C./90% RH) using various vacuum chamber deposition processes.
While one could employ traditional polymers, such as polypropylene or polyethylene, that exhibit good moisture barrier properties as an outer layer to improve the effectiveness of this barrier and thereby the quality of the product, such an incorporation would impact degradability. To retain degradability and quality, any other components must be degradable and commercially reasonable.
U.S. Pat. No. 5,153,074 describes the use of an extrusion-grade EVOH of typically 48% wt ethylene content coextruded with a maleic anhydride grafted propylene homopolymer or copolymer and biaxially oriented into a film. This film is then metallized on the EVOH surface for high barrier properties. However, such a formulation cannot be used in a coating process due to high ethylene content EVOH which cannot be dissolved in water. Nor is a biopolymer substrate such as PLA contemplated. In addition, the high ethylene content of the EVOH used prevents such a material from being biodegradable or compostable.
U.S. Pat. No. 5,175,054 describes the use of in-line coating between the MDO and TDO, a PVOH dispersion blended with a metal-containing ionic copolymer onto a biaxially oriented polymer substrate. The ionic copolymer acts as a tie-layer resin or primer to enable good adherence of the PVOH to the polyolefin substrate. This in-line coated film is then metallized via vacuum deposition on the PVOH blend surface. However, this reference is not a biopolymer-based substrate and would not exhibit biodegradable or compostable properties.
U.S. Pat. No. 4,464,438 describes the blend of PVOH and EVOH with a processing aid to enable extruding and stretching such a blend into a film. However, these blends were not coextruded or coated onto a BOPP substrate, nor is a biopolymer-based substrate contemplated.
U.S. Pat. No. 5,731,093 describes the use of in-line coating between the MDO and TDO, a PVOH blend with polyvinylidene chloride (PVdC) onto a multilayer biaxially oriented polypropylene film substrate. The PVOH blend with PVdC surface is then metallized via a vacuum deposition process. Excellent barrier properties are obtained, but the use of PVdC raises environmental concerns. Moreover, the polypropylene substrate would not be biodegradable.
U.S. Pat. No. 5,473,439 describes the use of crosslinked EVOH coatings on biaxially oriented polypropylene or polyethylene substrates. However, there is no indication of the efficacy of such coatings on PLA substrates or the use of combining EVOH with PVOH and crosslinking this blend. In addition, these pololefin substrates would not be biodegradable.
U.S. patent application Ser. No. 12/332,153 describes coextrusions of polyolefin metal receiving layers on a PLA core layer to improve moisture barrier properties after metallization. However, no coatings are contemplated to be applied directly to the PLA substrate.
PCT application PCT/US2009/54022 describes the improvement of moisture barrier properties on metallized PLA substrates via a unique process of sputter-deposited copper or other metal “seeding” or “priming” of the PLA substrate prior to aluminum vapor deposition. However, this process is not contemplated on a coating applied to the PLA substrate. This reference is incorporated in its entirety in this application.